The ability to reduce the surface tension of water is of great importance in the application of waterborne formulations because decreased surface tension translates to enhanced substrate wetting in actual formulations. Examples of such waterborne compositions include coatings, inks, adhesives, fountain solutions, cleaning compositions, metalworking fluids, and agricultural formulations. Surface tension reduction in water-based systems is generally achieved through the addition of surfactants, resulting in enhanced surface coverage, fewer defects, and more uniform distribution. Equilibrium surface tension performance (EST) is important when the system is at rest. Dynamic surface tension (DST) provides a measure of the ability of a surfactant to reduce surface tension and provide wetting under high speed application conditions.
Surfactants derived from amines such as N-methylglucamine are known for their equilibrium surface tension-reducing capabilities with few of the negative features of traditional nonionic and anionic surfactants.
The importance of a surfactant in achieving low surface tension at low use levels, the ability to affect foaming performance, and the importance of a surfactant in achieving efficient emulsification and solubilization is of considerable industrial importance and is well-appreciated in the art. The fundamental properties and the practical application of surfactants are described in more detail in Surfactants and Interfacial Phenomena, 2nd Ed. (Rosen) and in Kirk Othmer Encyclopedia of Chemical Technology, 4th Ed., Vol. 23, pp 477-541, which disclosures are incorporated herein by reference.
Many surfactants have the ability to emulsify or solubilize otherwise insoluble organic materials in aqueous media. This emulsification or solubilization occurs at concentrations higher than the critical micelle concentration (CMC). Thus it is desirable for surfactants to have low critical micelle concentrations since this will lead to more efficient surfactant utilization [Rosen, p. 171]. Low critical micelle concentrations are also important because they lead to diminished skin and eye irritation.
The ability of a surfactant to reduce the surface tension of an aqueous formulation is important in promoting substrate wetting. Two parameters that are important when evaluating the relative ability of a surfactant to provide surface tension reduction are the efficiency and effectiveness of the surfactant. The efficiency of a surfactant can be defined by its pC20 value:pC20=−logC20where C20 is the concentration in moles/liter of surfactant required to reduce the surface tension of water by 20 dynes/cm. pC20 provides a means for comparing the relative amount of surfactant required to obtain a given surface tension reduction. Since the scale is logarithmic, an increase in pC20 value of 1 corresponds to a decrease by a factor of 10 in the amount of surfactant required to provide a given surface tension reduction.
The effectiveness of a surfactant can be defined by its limiting surface tension (limiting γ) which is the minimum surface tension observed for an aqueous solution of the surfactant, regardless of surfactant concentration. Effective surfactants can provide wetting under challenging conditions such as those presented by high energy or contaminated substrates.
The foaming characteristics of a surfactant are important because they can help define applications for which the surfactant might be suitable. For example, foam can be desirable for applications such as ore flotation and cleaning. On the other hand, in coatings, graphic arts and adhesive applications, foam is undesirable because it can complicate application and lead to defect formation.
Although equilibrium surface tension reduction efficiency is important for some applications, other applications may require both equilibrium and dynamic surface tension reduction. However, the efficiency with which a surfactant will reduce equilibrium surface tension is not always proportional to the efficiency with which it will reduce dynamic surface tension. A typical example of this low equilibrium surface tension/high dynamic surface tension performance is observed for fluorosurfactants in Kissa's Fluorinated Surfactants, Surfactant Science Series, Volume 50, p. 126-7. The dynamic efficiency of a surfactant may be described in a manner similar to that of equilibrium efficiency:pD20(x)=−logD20(x)where D20 is the concentration in moles/liter of surfactant required to reduce the dynamic surface tension of an aqueous solution to 52.1 dynes/cm, or 20 dynes/cm below that of pure water when the measurement is performed using the maximum bubble pressure method at bubble rate x. Similar to comparisons between pC20 values, an increase in pD20(x) value of 1 corresponds to a decrease by a factor of 10 in the amount of surfactant required to provide a given dynamic surface tension reduction.
Low dynamic surface tension is of importance in the application of waterborne coatings. Schwartz, J. [“The Importance of Low Dynamic Surface Tension in Waterborne Coatings”, Journal of Coatings Technology, September 1992] discusses surface tension properties in waterborne coatings, including a discussion of dynamic surface tension in such coatings. Low dynamic surface tension is an important factor in achieving superior film formation in waterborne coatings and preventing defects such as retraction, craters, and foam.
Efficient application of agricultural products is also highly dependent on the dynamic surface tension properties of the formulation. In one article, [Wirth, W.; Storp, S.; Jacobsen, W. “Mechanisms Controlling Leaf Retention of Agricultural Spray Solutions”; Pestic. Sci. 1991, 33, 411-420], the relationship between the dynamic surface tension of agricultural formulations and the ability of these formulations to be retained on a leaf was studied. These workers observed a good correlation between retention values and dynamic surface tension, with formulations, which display more effective retention exhibiting low dynamic surface tension.
Low dynamic surface tension is also important in high-speed printing and is discussed in detail in “Using Surfactants to Formulate VOC Compliant Waterbased Inks” [Medina, S. W.; Sutovich, M. N. Am. Ink Maker 1994, 72 (2), 32-38]. Dynamic surface tension measurements provide an indication of the ability of the surfactant to migrate to a newly created ink/substrate interface to provide wetting during high-speed printing. U.S. Pat. No. 5,098,478 [Krishnan, et al.] teaches that dynamic surface tension in ink compositions for publication gravure printing must be reduced to a level of about 25 to 40 dynes/cm to assure that printability problems will not be encountered. U.S. Pat. No. 5,562,762 [Mrvos, et al.] teaches that low dynamic surface tension is important in ink jet printing.
Low dynamic surface tension is also important in various areas of industrial, institutional, and precision cleaning. The substrate to be cleaned needs to be wetted in order for the soil to be lifted up and separated from the substrate. A similar wetting is necessary for oil and gas applications where oil needs to be separated and removed from narrow pores and cracks in soil and rock.
WO 9519951A1 discloses polyhydroxydiamine compositions having the following structure: where R1 and R2 are hydrogen, substituted or unsubstituted alkyl, aryl, or alkylaryl groups, X is a bridging group with from 2-200 atoms, and Z1 and Z2 are the same or different alcohol-containing moieties with one or more hydroxyl groups. The compounds are said to be useful in laundry, cleaning, and fabric and personal care compositions.
WO 9519953A1 discloses gemini polyhydroxy fatty acid amides having the following structural formula: R1 and R2 are hydrocarbyl moieties having 1-21 carbon atoms, X is a bridging group with from 2-200 atoms, and Z1 and Z2 are the same or different alcohol-containing moieties with one or more hydroxyl groups. Although examples of application formulations are provided, there is no discussion of the performance of these molecules.
Tenside Surf. Det., 2001, 38(1), 7-14 [Warwel] describes two families of glucamine-epoxide adducts. Although some molecules are said to exhibit low equilibrium surface tension (C10 and C12), the molecules are monomeric, not highly soluble, and have varying foam characteristics. 
Sugar-based tertiary amino gemini surfactants are described in Eur. J. of Biochem. 2001, 268(5), 1269-79 and are represented by the formula: n is 4 or 6 and R is represented by C12, C14, C16, and C18 saturated or C18 (oleyl) unsaturated alkyl moieties. Although the equilibrium surface tension is described for these surface-active agents, these molecules are not efficient in their equilibrium surface tension reduction.